Air spaced coaxial high-frequency cable



1940- E. FISCHER El AL 2.210.400

AIR SPACED COAXIAL HIGH-FREQUENCY CABLE Filed April 27, 1937 2 Sheets-Sheet l Our-5,3 Wax/9270 v wsz/mmva Coma/area Jame/e POZZz'Cf/V Jase/w fl'gf? CONDUCTO/ES g- 6, 1940- E. FISCHER ET AL 2.2l0,400

AIR SPACED COAXIAL HIGH-FREQUENCY CABLF Filed April 27, 1937 2 Sheets-Sheet 2 do/voucraes 3 app/ 54 l V/ND/NG 97 Can 000v? v 7/ Patented Aug. 6, 1940 UNITED STATES AIR SPACED COAXIAL HIGH-FREQUENCY CABLE Ernst Fischer,

Berlin Lichterfelde,

Johannes Fischer and Wilhelm Gabriel, Berlin-Spandau,

Hermann Lintzel,

Berlin-Siemensstadt, and

Herbert Menke, Berlin-Spandau, Germany, assignors to Siemens & Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application April 27, 1937, Serial No. 139,200 In Germany April 27, 1936 10 Claims.

Our invention relates to air spaced coaxial high-frequency cables and has for its object the improvement of the properties of transmission and the simplification of the manufacture of such cables as a result of a novel design of the outer conductor.

With respect to the construction of the outer conductor of concentric high-frequency conductors various proposals have already been made. According to an earlier known proposal the outer conductor is to be made of short tubular members each of which consists of'two half shells and which engage one another at their ends in the manner of a ball and socket joint in order to make the outer conductor flexible. Within this joint a spacing disc is placed through a central bore of which the inner conductor extends. Such outer conductors provided with ball joints are, however, tedious and expensive to make and have in addition the disadvantage of the presence of transmission resistances at the ball joints. On account of these disadvantages the procedure has generally been adopted in practice of making the outer conductor in the form of a stranded layer of electrically good conducting bands, the necessary flexibility being ensured by the use of a plurality of bands and by the selection of a sufficiently small length of lay thereof. The stranding of the outer conductor, however, causes, on

3 the one hand an increase in the longitudinal resistance and on the other hand the creation of longitudinal magnetic fields. Furthermore, outer conductors stranded from a plurality of t bands present difiiculties in obtaining a diameter which is constant over the entire length of the conductor.

It is also known to make the outer conductor of a single strip bent into tubular form and to fold the longitudinal edges into aseam when, if necessary, the outer conductor so formed may be provided at intervals with annular grooves. This folding together of the longitudinal edges of the strips, has, however, the disadvantages that the diameter of the outer conductor is undesirably enlarged and the manufacture presents many difficulties. In particular the folding of these longitudinal edges can hardly be used in practice with air-spaced coaxial conductors primarily owing to the presence of the sensitive air-space- 00 insulation.

Furthermore, it has been proposed to manufacture the outer conductor of a coaxial highfrequency cable of a single strip bent into tubular form, the longitudinal edges of which overlap 55 one another or are soldered or welded together,

and the necessary flexibility of the outer conductor is to be obtained by closely spaced grooves. The arrangement of such grooves has, however, various drawbacks, for example, an increased consumption of material and a considerable in- 5 crease of the electric capacity and the longitudinal resistance.

The invention eliminates the aforementioned drawbacks by employing a novel shape of the outer conductor for air spaced coaxial highfrequency cables which shape presents technical advantages from the transmission point of view and which may be manufactured in a simple and economical manner. According to the invention the outer conductor is made of two, three or four strips of arcuate cross-section which are provided at intervals with circular grooves deep in proportion to the diameter of the outer conductor and which preferably extend without twist in the longitudinal direction. Of particular advantage is the use-of only two strips of semicircular cross-section so as to give the strips the shape of bearing brasses. However, it may also be advantageous to employ three or four'strips of arcuate cross-section. The annular grooves are preferably made of a depth such that the diameter at the deepest point of the groove amounts to less than 90% of the diameter of the outer conductor. It has been found advisable to make the smallest diameter of the annular grooves about equal to 70-80% of the diameter of the outer conductor.

According to the invention the annular grooves arranged at intervals may be used at the same time to hold firmly in position the insulating bodies arranged in the interior of the outer conductor, the inner conductor extending through a central hole in these bodies, so that they serve at the same time as spacing members for the inner conductor. These insulating bodies are held in position by the inner peripheral ribs produced by the circumferential grooves engaging in annular grooves provided on the respective insulating bodies, these grooves being made so that the said inner peripheral ribs have suflicient freedom of movement within the grooves in the bodies. It is, however, particularly advantageous to arrange pairs of annular grooves close together in the tubular body and to place a preferably disc-shaped insulating body between such a pair of grooves.

For simplifying the production of the coaxial cable, the shaping and the grooving in the transverse direction of the strips as also their assembly into the tubular outer conductor with the 56 simultaneous introduction of the insulating bodies is preferably carried out in a single operation. To this end, the strips running off supply drums preferably pass through profile rolls which give the strips the desired shape. It is, however, obviously also possible to make the outer conductor strips in a separate operation.

The invention is illustrated in the accompanying drawings, in which:

Figs. 1 and 2 represent in perspective two different forms in which the invention may be reduced to practice.

Figs. 3 and 5 represent in perspective further modified forms of the cable.

Fig. 4 represents'in perspective a semi-circular spacing disc element used in the modifications Figs. 3 and 5.

Fig. 6 is a transverse section'through the cable on line BB Fig. 3.

Fig. 7 is a transverse section through a modified form of Fig. 3, taken on a line similar to B--B in Fig. 3.

Figs. 8, 9, l0 and 11 show modified forms of additional insulation between the inner and outer conductors.

Fig. 12 shows in perspective a modified form of coaxial conductor in which also the inner conductor is made of sheet material and also longitudinally divided into two complementary halves, and

Fig. 13 shows in perspective a further modified form of coaxial conductor.

Referring to Fig. l, the coaxial cable is formed of an inner conductor and an outer conductor II which latter consists of two strips bent into semi-circular shape. The outer conductor, in order to increase its flexibility, is provided at short intervals along its length with annular grooves l2. These grooves produce inner peripheral ribs which are utilized for holding in position insulating bodies l3 provided with a central hole, through which the inner conductor l0 extends, so that the insulating bodies l3 simultaneously serve for centering the inner conductor within the outer one. The insulating body l3 has an external circumferential groove l4 into which the internal rib formed by the annular groove l2 engages. Thus the spacing of the annular grooves I2 corresponds to the spacing of the insulating bodies l3. The two strips of the outer conductor are held together by an externally applied strong band winding I 5. It is, however, also possible to join and hold the two outer conductor strips together in a different way, for example, by soldering or welding their abutting edges together. Over the coaxial cable so produced are applied in known manner a closed tape winding it of insulating material and a watertight cable sheath H.

The embodiment shown in Fig. 2 difiers from that of Fig. 1 only by arranging the annular grooves in the tubular conductor in closely adjacent pairs and'by placing an insulating body between the members of each pair of annular grooves. represents the inner conductor and 2| the outer conductor consisting of two strips bent into semi-circular form. At short intervals the outer conductor is provided with pairs of annular grooves 22 between which is arranged the disc-shaped insulating body 23, which at the same time serves as spacing member for the inner conductor 20. The two outer conductor ing 24. 25 indicates a closed winding of insulating material and 26 a watertight cable sheath.

The insulating bodies held by the annular grooves must consist of insulating material of suflicient mechanical strength. Where the coaxial cable is used for high-frequency transmission, insulating materials are preferably used which have a small dielectric constant and at high-frequencies a small phase angle difference. Ceramic products with low losses or insulating materials of the group of the polyvinyl compounds, such as polystyrene, should primarily be used.

The two embodiments described show the use of the invention for high-frequency cables with a single coaxial inner conductor. Naturally the invention may also be applied to high frequency cables having a plurality of concentric inner conductors as also with concentric conductors for other transmission purposes.

Within the scope of the present invention some other forms are hereinafter described which may be employed particularly in that type of coaxial cable in which the outer conductor consists of two strips bent in semi-circular shape. These forms of the invention are characterized by the employment of at least two insulating bodies for spacing the inner conductor within the outer conductor, one of the insulating bodies being se cured by additional means to onestrip of the outer conductor and the other to the other strip.

In this construction the semi-sleeve-like form of the individual outer conductor strips is utilized to facilitate the fixing of the insulating bodies the form of which is suitably adapted to the semicircular form of the strips so that the insulating bodies inserted in the concave side of each strip have a certain natural adhesion or grip. Only slight additional measures are then necessary to fix completely the insulating bodies on the conductor strips; for instance, simply gluing the insulating bodies to the outer conductor strips is sufiicient. It is, however, preferable to fix the insulating bodies by means of inwardly directed turned over portions or indentations provided on the longitudinal edges of the conductor strips.

An important advantage of this embodiment of the invention resides in the fact that the manufacture of the coaxial cable is facilitated and cheapened. The time hitherto absorbed in fitting of the spacing members is eliminated as it is unnecessary either to thread the spacingmembers on to the inner conductor or to fit them on laterally. The spacing members are attached to the outer conductor strips before or during'the manufacture of the coaxial cable. Further advantages are-presented when the outer conductor is made self-supporting. 'In this case the insulating bodies fixed on the outer conductor strips may be made comparatively thin as they serve simply for centering the inner conductor and not for supporting the outer conductor strips. This results in a reduction of the capacity and of the dielectric losses of the line. Furthermore, the speed of the electrical transmission is thereby increased.

The arrangement and construction of the insulating bodies may be carried out in various ways. A suitable way consists in fixing an insulating body of semi-disc or approximately semi-disc shape at the different spacing points to each outer conductor strip so that at each spacing point two insulating bodies will be located one opposite the other when the two halves are assembled into a cable. Alternatively an insulating body of semi-disc form may be fixed alternately on the one and on the other outer conductor strip at the successive spacing points distributed along the length of the cable. With this arrangement the spacing ofthe inner conductor is carried out alternately by consecutively following insulating bodies. In a further and similar embodiment known spacing discs are used which have a radial slot, for receiving the inner conductor. In this case a circular insulating body having a radial slot is arranged at each of the successive spacing points and alternately attached to one and then to the other outer conductor strip in such a manner that the radial slot in each insulating body extends at right angles to the dividing plane between the two outer conductor strips. If the insulating bodies are fixed in the annular grooves or between two closely adjacent annular grooves of'the outer conductor strips, the said bodies are preferably given the form of half discs. If the insulating bodies are fastened in the smooth portion of the outer conductor strips between two consecutive annular grooves, the insulating bodies may be given the form of a semi-cylindrical shell with transverse walls arranged at both ends, these end walls being provided in the center with a semicircular recess for receiving the inner conductor. Furthermore, the insulating bodies may be in the form of a continuous spacing element and may be secured to the outer conductor strips. It may be advantageous within the scope of the invention to secureadditional insulating bodies on the outer conductor strips in addition to the insulating bodies serving as spacing members, these additional bodies serving simply as a means of protection against contact between inner and outer conductors. Figures 7 to 9 of the drawings show suitable embodiments of this arrangement and will be described below.

According to a further feature of the invention the inner conductor of the coaxial cable is also divided longitudinally into two component conductor strips, and these strips may be separatelyunited with the insulating bodies secured to the individual strips of the outer conductor so that the corresponding strips of the two conductors are also united. To this end preferably the two component conductors of the inner conductor are provided at intervals with annular grooves. By firmly securing the component conductors of the inner conductor with the semicircular shaped outer conductor strips, the particular advantage is obtained that the distance of the inner conductor from the outer conductor is, maintained constant over the whole length even in cases of bending or torsional stressing of the cable; this is of particular importance in avoiding characteristic impedance fluctuations within the manufactured cable lengths.

The above-described, embodiments of the invention are shown in Figures'3 to 12 of the accompanying drawings which will now be re ferred to.

According to Fig. 3 a concentric cable consists of an inner conductor 30 and an outer conductor formed of two strips 39 and 32 of semi-circular cross-section. The outer conductor strips are provided at short intervals with pairs of closely adjacent annular grooves 33, and between individual grooves of each pair a spacing member 35 is secured for centering the inner conductor 30. The spacing members 3% have the form oi half'discs as shown clearly in Fig. 4. As the shape of the spacing members 343 is adapted to the shape of the circular groove it is fairly fixed between the pair of annular grooves without any additional -means being necessary. Point-like indentations 35 at the longitudinal edges of the outer conductor strip, however, ensure a complete fixing of the spacing member within the concave portions of the conductor strip so as to avoid with absolute certainty the possibility of the spacing member falling out when joining the outer conductor strips together to form the finished conductor. The two outer conductor strips are held together by a band winding 36 which consists of a strong band, for example, of copper. An insulating band winding 37 and a watertight cable sheath 38 are then applied thereto.

The embodiment shown in Fig. differs from that of Fig. 3 mainly only by displacing in the two outer conductor strips the semi-annulan groove portions in longitudinal direction, so that a circumferential groove in each strip becomes located longitudinaiiiy intermediate adjacent grooves in the other conductor strip. The individual parts in Fig. 5 which correspond with those of Fig. 3 are provided with the same reference numerals. The shifting of the annular grooves of the two conductor strips with respect to each other has the advantage that the longitudinal edges of the two strips support one another and cannot slip over one another in the direction of their plane of abutment. Fig. 6 shows a cross-section of the coaxial cable made in accordance with Fig. 3, on the line B-B of the latter figure.

Fig. '7 shows a coaxial cable in which in addition to thetransverse insulating bodies serving as spacing pieces, additional insulating bodies are provided which serve simply as means for affording protection against contact between the conductors. In Fig. 7 it is assumed that the annular grooves of the two outer conductor strips register as shown, for example, in Fig. 3. The cross-sectional surface shown in Fig. 7 lies, therefore, at a point of the concentric, cable which corresponds in position to the sectional line B-B of Fig. 3. The inner conductor of the coaxial cable is indicated at 39. At each spacing point an insulating body 40 is provided which serves as a spacing member for the inner conductor and has the form of a half-disc attached to one outer conductor strip while an additional insulating body Bi in the form of a half ring is provided in the groove of other outer conductor strip as a protection against contact between the conductors. The two outer conductor strips are indicated at 42 and it. Over the outer conductor two band windings 46 and 55 are applied and then a cable sheath it. The conductor as= sembly is so built up that there is mounted on each outer conductor strip at the successive spacing points, alternately a spacing member ill and a contact-preventing body ii, the elements 60 and it being displaced respectively in the two conductor strips in such a manner that at each spacing point, a spacing member in one conductor strip registers with a contact-preventing body in the other strip.

Fig. 8 shows a portion of an outer conductor strip in which a. supplementary contact-preventing bocly is secured in the smooth inner cylindrical portion of the conductor strip between adiacent pairs of the annular grooves. Each outer conductor strip M is provided with pairs of closely adjacent grooves of which one pair is indicated at it. Between the adjacent grooves dd of each pair a spacing member 59 is fixed in the manner described with reterence to preceding examples. Located against the smooth inner cylindrical surface of each outer conductor strip is a semi-cylindrical body of sheet insulating material which is secured to its appertaining conductor strip by means of detents 5| cut out of the outer conductor edge and which overlap the edges of the body 50. The form of the contact-preventing body 50 shown in Fig. 8 is indicated more clearly by Fig- 9 which shows it separately. Such a contact-preventing body may have comparatively thin walls so that the electric capacity is only slightly increased by such supplementary contact-preventing bodies. The cable containing the structure shown in Fig. 8 is otherwise constructed as apparent from Fig. 10 shows an insulating body similar to that of Fig. 9; and which is also secured in the smooth portions of the outer conductor strips between adjacent pairs of annular grooves. As is clear from Fig. 10 the insulating body is in the form of a half sleeve with semi-circular transverse walls at both ends, these end walls having semi-circular recesses 52 to accommodate the inner conductor.

Differing from Fig. 10, Fig. 11 shows an insulating body in the form of an open string winding of generally semi-circular contour and consisting of a solid insulating material and in the diametrical portions of which indentations 53 are provided to carry the inner conductor. Such an insulating body is secured between two consecutive annular groups of grooves of an outer conductor strip, such as between successive groups 48 in Fig. 8, to be located similar to elements 50 in that figure, preferably by inwardly upsetting portions of the longitudinal conductor edges similar to the manner shown at 5| in Fig. 8. The production of this form of insulating body is preferably effected by continuous production of the string winding and subsequent cutting it up into pieces of prescribed length. Before inserting the insulating bodies so made into place in the outer conductor strip they are preferably mixed up so that their consecutive arrangement in the conductor is different from the one in which they were cut from the continuous string winding whereby any unevenness in the diameter of the string and of the entire string winding is to a large extent compensated.

Fig; 12 shows an embodiment in which the inner conductor also is divided into two complementary strips, the figure shows one half of the entire concentric conductor in perspective. The outer conductor strip 54 is provided in the manner already described above with annular grooves 55 spaced at short intervals apart, these grooves not only serving to increase the flexibility but also for securing the insulating bodies 56. The complementary conductor elements such as 51 of the inner conductor are also formed of strips bent into a semi-cylindrical sleeve-like shape which at short intervals apart are provided on both sides of the insulating body 56 with external peripheral annular ridges 58 so that the complementary conductor strips 51 are held in the semi-circular recesses formed in the insulating bodies 56. By additional means, for example, by point-like indentations59, the complementary conductor strips 51 are securely fixed on the insulating bodies 55. The two strips of the coaxial cable are made identical and they are then placed one on the other so as to form the concentric double conductor in which the annular grooves coincide similarly to the manner shown in Fig. 3 or are staggered with respect to each other in the longitudinal direction in the manner indicated in Fig. 5. In order to avoid lateral'shifting of the two halves of the coaxial cable in the diametrical plane of abutment, a core in the form of a solid wire or a cord or a helical spring of insulating material or a suitable metal may be arranged in the center of the inner conductor.

With the concentric cable made according to the invention the strips bent into semi-sleeve- .like form extend in the longitudinal direction without twistin order to avoid the creation of longitudinal magnetic fields. In particular cases, however, the strips may be given a very long lay to lessen the stresses on the annular grooves on bending the cable, without thereby sufiiciently increasing the longitudinal magnetic fields to have a disturbing efiect. The hollow conductor, namely both the outer conductor and the inner conductor may be made, instead of two, of three or four strips shaped in a corresponding manner. The invention may also be used in the manufacture of air spaced twisted double or spiral quad cables which are surrounded with a screen of good electrical conductivity extending in the longitudinal direction of the cable, the screen itself forming the outer conductor.

A further advantageous form of the invention consists in making the outer conductor of an air spaced coaxial high-frequency cable of a plurality of metal walls of arcuate cross-section combined to form a tubular conductor, the strips being provided at short intervals with comparatively deep annular grooves, the bases of which' are supported on the air-space-insulation which surrounds the inner conductor throughout its length. y Y

The comparatively deep annular grooves provided at short intervals not only ensure in this case the flexibility required for the outer conductor, but also serve at the same time as spacing members for the air-space-insulation so that the consumption of solid insulating materials for the air-space-insulation is considerably reduced by the presence of the annular grooves.

Such an embodiment is shown in Fig. 13.

The inner conductor 60 is first surrounded witha winding of a continuous spacing element 6| in open helical turns and is then surrounded with a closed tape winding 62 ofinsulating malength with comparatively deep annular grooves insulating material and a watertight cable sheath 61. The air-space-insulation between the inner In Fig. 13 an embodiment of a high-frequency cable is shown which contains only a single concentric conductor; The concentric cable thus made may naturally be used also for building up cables having a number of concentric conductors. Furthermore, one or more concentric conductors may be combined with other communication lines, for instance. with stranded pairs and quads. into composite cables.

We claim as our invention: 7

l. A high-frequency cable with coaxial air spaced conductors, comprising an outer conductor composed of a plurality of continuous conductive strips of arcuate cross section, said strips having annular grooves arranged at intervals larger than the width of said grooves relative to the axial direction of the'cable, said grooves projecting inwardly and having suflicient depth to render the conductor flexible at said grooves, and means cooperating with said grooves for maintaining the air spacing between said conductors.

2. A high-frequency cable with coaxial air spaced conductors, comprising an outer conductor composed of a plurality of continuous conductive strips of arcuate cross section, said strips having inwardly projecting grooves of sufficient depth to render said outer conductor flexible at said grooves, said grooves extending from one to the other longitudinal edge of said strips and being arranged without longitudinal twist at intervals greater than the width of said grooves, and means cooperating with said grooves for maintaining the air spacing between said conductors.

3. A high-frequency cable with coaxial air spaced conductors, comprising an outer conductor composed of a plurality of continuous conductive strips of arcuate cross section, said strips having groups of inwardly projecting annular grooves of suificient depth to render the conductor flexible, said groups being spaced axially at intervals larger than the width of the individual grooves of each group, said individual grooves being spaced at short intervals within each group.

4. A high-frequency cable having at least two air spaced coaxial conductors, at least the outer conductor being composed of a plurality of continuous conductive strips of arcuate cross section, said strips having annular grooves arranged at intervals larger than the width of said grooves relative to the axial direction of the cable, said grooves projecting inwardly and having suflicient depth to render the conductor flexible at said grooves, and insulating bodies arranged within said outer conductor and held in place by said grooves, said bodies filling only a fraction of the interior space of said outer conductor, and said inner conductor passing centrally through said bodies.

5. A high-frequency cable having at least two air spaced coaxial conductors, at least the outer conductor being composed of a plurality of continuous conductive strips of arcuate cross section, said strips having pairs of inwardly projecting annular grooves spaced at a short interval from each other and having suflicient depth to render the cable flexible at said grooves, said pairs being arranged at intervals larger than the width of each pair, and insulating bodies spaced inside said outer conductor and centrally traversed by the inner conductor, each individual insulating body being positioned between the inner peripheral ridges formed by a pair of said annular grooves.

6. In a high-frequency cable having a central conductor and a coaxial outer conductor, said outer conductor consisting of two continuous conducting strips of semi-circular cross section, said strips being provided with groups of inwardly turned grooves spaced at intervals longitudinally of the cable and of suflicient depth to render the cable flexible at said grooves, and a plurality of insulating bodies of approximately semi-circular cross section arranged inside said outer conductor, each of said insulating bodies being arranged between adjacent grooves of one of said groups for spacing said central conductor with respect to said outer conductor.

'7. A high-frequency cable having at least two coaxial conductors, at least the outer conductor consisting of two conducting strips of semi-circular cross section, said strips being provided with annular grooves spaced at intervals longitudinally of the cable, and a plurality of insulating bodies of semi-circular .cross section arranged inside the outer conductor, said insulating bodies being alternately secured in the one and in the other outer conductor strip at successive groove spacing points, for spacing the inner conductor with respect to the outer conductor.

8. A high-frequency cable having at least two coaxial conductors, the outer conductor consisting of two conducting strips of semi-circular cross section, said strips being provided with groups of inwardly turned closely adjacent peripheral grooves spaced at intervals longitudinally of the conductor and of sufiicient depth to render the cable flexible at said grooves, a plurality of insulating bodies having a sectorshaped cross section covering only part of the cross section of said outer conductor, said bodies being spaced along the inside of the outer conductor, a portion of the insulating bodies being positioned by the grooves of and secured to one outer conductor strip, and the other portion or the insulating bodies being positioned by said grooves of and secured to the other outer conductor strip, the peripheral grooves of one outer conductor strip and, therefore, also the insulating bodies secured to the strips, being displaced in the longitudinal direction with respect to the grooves and insulating bodies of the other conductor strip.

9. A high-frequency cable having at least two air spaced coaxial conductors, the outer conductor consisting of two conducting strips of semi-circular cross section, said strip being provided at intervals with a group of inwardly turned closely spaced circumferential grooves of sufficient depth to render the cable flexible at said grooves, insulating material disposed inside of each outer conductor strip and positioned by its grooves for spacing the inner conductor at the groove intervals from the outer conductor and of the outer conductor and positioned between adjacent groups of grooves for preventing accidental contact between said conductors intermediate the groups of grooves, the spacing insulating material of one conductor strip being longitudinally displaced with respect to that of the other strip so that the spacing portions of the insulating material of each strip are located opposite the contact preventing portions of the material of the other'strip.

ERNST FISCHER. JOHAN'NES FISCHER. WILHELM GABRIEL. HERMANN LINTZEL. HERBERT MENKE. 

